FIG. 1 is a block diagram illustrating selected components of an information storage system (disk drive) 110 according to the prior art. Disk drives have one or more disks 111 on which ferromagnetic thin materials are deposited. The disk drive includes data recording disk 111, pivoting actuator arm 113, and slider 31 that includes a read head and a write head (not shown). The functional blocks include servo system 90, read/write electronics 114, interface electronics 115, controller electronics 116, microprocessor 117, and RAM 118. A disk drive can include multiple disks stacked on a hub that is rotated by a disk motor, with a separate slider for each surface of each disk. The term servo wedge 120 will be used to mean the contiguous set of servo fields extending from ID to OD on the disk.
Disk 111 will typically have multiple servo wedges 120 arranged radially around the disk, but only two are shown for simplicity. Information recorded on the disks is generally organized in concentric tracks or, alternatively, the tracks can be arrange in a plurality of spiral tracks. (For a description of spiral tracks see, for example, U.S. Pat. No. 7,113,362 Lee, et al. Sep. 26, 2006.) In embodiments either of these tracks organizations can be used, and the term “tracks” will be used generically to include these any other similar forms of arrangement.
As part of the manufacturing process permanent servo information is recorded on the disks that provides information to the system about the position of the heads when the disks are rotating during operation. The servo identifier (SID) data on the disk provides several fundamental functions and is conventionally arranged in four distinct fields in each of the plurality of servo sectors angularly spaced around the disk. FIG. 2B illustrates the fields in a selected servo ID (SID) 20 according to the prior art. The preamble precedes servo address mark (SAM) which is a timing mark which is used to synchronize data within the servo fields, and also provides timing information for write and read operations in the data portions of the disk. Second, the SID track-ID (TID) number and additional information to identify the physical SID number. The servo field supplies a multi-bit digital track-ID (TID) number, which is typically written in Gray code. During seek operations, when the head is moving across tracks, the head can typically only read a portion of the Gray-code in each TID. Therefore, the Gray-code is constructed so that pieces of the TID, in effect, can be combined from adjacent tracks to give an approximate track location during a seek.
The SID also supplies a position error field (A & B bursts in this example), which provides the fractional-track Position Error Signal (PES). A repeatable run-out (RRO) field follows the PES bursts in this example. During read or write operations the drive's servo control system uses the PES servo information recorded on the disk surface as feedback to maintain the head in a generally centered position over the target data track. The typical PES pattern includes a burst pattern in which the bursts are identical sets of high frequency magnetic flux transitions. Unlike the track-ID (TID) field number, the PES bursts do not encode numerical information. In contrast to the TID, it is the position of the bursts that provide information on where the head is relative to the centerline of a track.
Each of these servo functions typically consumes a relatively independent portion of the servo wedge in conventional servo systems. Typically, the servo fields consume a significant portion of the recording surface of the disk and are, therefore, an attractive target for reduction.
The Integrated Servo concepts which are referenced herein are described in published U.S. patent applications:                20110149434 by Coker, et al. (pub. Jun. 23, 2011), Ser. No. 12/653,874, filed Dec. 18, 2009;        20110149433 by Coker, et al. (pub. Jun. 23, 2011), Ser. No. 12/653,863, filed Dec. 18, 2009; and        20110149432 by Coker, et al. (pub. Jun. 23, 2011), Ser. No. 12/653,862, filed Dec. 18, 2009.        
Embodiments of the Integrated Servo concept implement some or all major servo subfunctions for a storage device in Integrated Servo fields comprising sequences of encoded bits having selected mathematical properties. The Integrated Servo field is composed of one or more encoded sequences, which are members of a selected allowable sequence set that is constrained to provide some or all of the following functions: the Servo Address Mark (SAM), the Position Error Signal (PES) and higher level positional information such as the track-ID. The Integrated Servo fields can also provide a fractional Position Error Signal (PES) in relation to the center of a data track through the relative amplitude of the signal read for adjacent sequences disposed laterally across the tracks. The servo system detects the sequences in the signal from the read head using a set of digital filters corresponding to the set of encoded sequences. Embodiments of Integrated Servo constraint the placement of sequences so that only mathematically orthogonal sequences are placed next to each other on adjacent tracks.
As illustrated in FIG. 2A a write-to-read gap 33 is included in the track structure to allow for the physical separation between the write head 32 and the read head 33 in slider 31 and to provide the time/distance needed to switch from writing data to reading the next servo sector ID (SID) 20. The servo gate assertion period (window) 25A begins in the preamble and ends in this example with the RRO field. The bulk of the write-to-read gap is caused by the physical separation between the writer and reader in the slider. In most head designs the reader leads the writer as shown, so when the writer reaches the end of the data sector, the reader is already some distance beyond the end of the data sector which creates a physical gap. In addition some gap is needed to allow for the time needed for the drive's control systems to switch from writing to reading, but this switching gap is much smaller than the physical writer-to-reader separation. Accordingly servo systems have typically included a write-to-read gap 33 in the track format between the end of a writable data sector and the start of the following servo sector information.